1. Field of the Invention
The present invention relates to a plasma processing apparatus and a method of plasma distribution correction and, more particularly, to a plasma processing apparatus having a focus ring.
2. Description of the Related Art
A plasma processing apparatus for performing a plasma processing on a wafer serving as a substrate using plasma is provided with a housing chamber (chamber) for housing wafers, a mounting stage disposed within the chamber to be mounted with a wafer, and a focus ring disposed on the mounting stage so as to surround the wafer. The focus ring is made of silicon and converges ions in plasma within the chamber onto the wafer, in order to improve the efficiency of a plasma processing, for example, an etching processing, to be performed on the wafer.
In addition, as shown in FIG. 11A, a focus ring 110 maintains a potential difference between the focus ring 110 and plasma 111 the same as a potential difference between a wafer W and the plasma 111. Thus, a sheath 112 formed between the wafer W (or focus ring 110) and the plasma 111 is kept level. As a result, all of ions 113 passing through the sheath 112 almost perpendicularly enter the wafer W, thereby forming vertical trenches and holes across the wafer W.
However, since the focus ring 110 is made of silicon and is therefore abraded by plasma, the upper surface of the focus ring 110 may become lower than the surface of the wafer W. At this time, the plasma 111 facing the upper surface of the focus ring 110 is displaced (lowers) according to the lowering of the upper surface. Consequently, the sheath 112 may become distorted around the peripheral part of the wafer W (see FIG. 11B). Ions 113 having passed through the distorted sheath 112 obliquely enter the peripheral part of the wafer W, thereby forming trenches and holes at a tilt. In addition, since some ions 113 obliquely enter the wafer W, the density of ion entry across the wafer W is no longer constant. As a result, an etch rate becomes uneven across the wafer W.
Hence, there has heretofore been developed a plasma etching apparatus for preventing sheath distortion by mechanically raising a worn focus ring (see, for example, Japanese Patent Laid-Open No. 2006-173223). This apparatus measures the upper surface position of the focus ring using laser light to determine the amount of wear in the focus ring, thereby adjusting the upper surface position of the focus ring based on the amount of wear.
However, the above-described apparatus requires a drive unit having a shaft and a motor in order to mechanically raise the focus ring, thus giving rise to the problem that the configuration thereof becomes complicated. The apparatus has another problem that when the focus ring moves, deposits adhering thereto separate on impact of the movement and fly apart in particles, thus attaching to a wafer.